A projection exposure apparatus for EUV microlithography is designed for an operating wavelength in the extreme ultraviolet and thus below approximately 100 nm. In particular, the operating wavelength can be 13.5 nm or 7 nm. Depending on the availability of light sources and imaging optical elements, however, it is also possible to use other operating wavelengths.
A projection exposure apparatus of this type usually has an illumination system and a projection objective. The illumination system is used to generate a desired light distribution for illuminating a pattern of a mask or a reticle. The projection objective is used to image the illuminated pattern onto a light-sensitive material with extremely high resolution and thereby to expose the light-sensitive material with the pattern in a structured manner. On the basis of the pattern exposed into the light-sensitive material, real structures can be produced in a semiconductor material, for example, by subsequent work steps.
In order to obtain a high resolution, the projection objective should have a high numerical aperture. Since a projection objective for the EUV range has mirrors as imaging optical elements, it is relatively difficult to realize a high numerical aperture. One option is to provide an (in particular central) obscuration of a pupil plane of the projection objective. As a result, it is possible, for example, to lead the imaging beam path through a perforation in one or in a plurality of imaging optical elements. However, the obscuration can cause problems because only a partial region of the pupil plane contributes to imaging.